At the end of Kingdom Hearts II, there was a small teaser for Kingdom Hearts III. That's literally all we've had to go on for all these years. Square Enix has produced a number of spin-offs of the series, but seemed to have ditched the third real installment.
Finally, after all this time, the first details for Kingdom Hearts III have trickled out thanks to Famitsu magazine.
Tetsuya Nomura has said that Kingdom Hearts III will be the last game for main antagonist, Xehanort. However, it's not the end of the franchise, as the framework is already in place for future installments, all putting Sora as the lead protagonist.
He also said to look for answers to KHIII's plot in the upcoming Kingdom Hearts 3DS, which is yet another spin-off.
Nomura also mentioned that Square Enix is looking into the tech required to render past titles in high-definition, which hints at Square Enix jumping on the HD remaster collection bandwagon.